diff --git a/library/std/src/ffi/c_str.rs b/library/std/src/ffi/c_str.rs
index 6e70d5ca02d75..1678367290e51 100644
--- a/library/std/src/ffi/c_str.rs
+++ b/library/std/src/ffi/c_str.rs
@@ -382,7 +382,7 @@ impl CString {
                 let bytes: Vec<u8> = self.into();
                 match memchr::memchr(0, &bytes) {
                     Some(i) => Err(NulError(i, bytes)),
-                    None => Ok(unsafe { CString::from_vec_unchecked(bytes) }),
+                    None => Ok(unsafe { CString::_from_vec_unchecked(bytes) }),
                 }
             }
         }
@@ -405,7 +405,7 @@ impl CString {
             // This allows better optimizations if lto enabled.
             match memchr::memchr(0, bytes) {
                 Some(i) => Err(NulError(i, buffer)),
-                None => Ok(unsafe { CString::from_vec_unchecked(buffer) }),
+                None => Ok(unsafe { CString::_from_vec_unchecked(buffer) }),
             }
         }
 
@@ -451,10 +451,15 @@ impl CString {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub unsafe fn from_vec_unchecked(mut v: Vec<u8>) -> CString {
+    pub unsafe fn from_vec_unchecked(v: Vec<u8>) -> Self {
+        debug_assert!(memchr::memchr(0, &v).is_none());
+        unsafe { Self::_from_vec_unchecked(v) }
+    }
+
+    unsafe fn _from_vec_unchecked(mut v: Vec<u8>) -> Self {
         v.reserve_exact(1);
         v.push(0);
-        CString { inner: v.into_boxed_slice() }
+        Self { inner: v.into_boxed_slice() }
     }
 
     /// Retakes ownership of a `CString` that was transferred to C via
@@ -578,7 +583,7 @@ impl CString {
     pub fn into_string(self) -> Result<String, IntoStringError> {
         String::from_utf8(self.into_bytes()).map_err(|e| IntoStringError {
             error: e.utf8_error(),
-            inner: unsafe { CString::from_vec_unchecked(e.into_bytes()) },
+            inner: unsafe { Self::_from_vec_unchecked(e.into_bytes()) },
         })
     }
 
@@ -735,6 +740,11 @@ impl CString {
     #[must_use]
     #[stable(feature = "cstring_from_vec_with_nul", since = "1.58.0")]
     pub unsafe fn from_vec_with_nul_unchecked(v: Vec<u8>) -> Self {
+        debug_assert!(memchr::memchr(0, &v).unwrap() + 1 == v.len());
+        unsafe { Self::_from_vec_with_nul_unchecked(v) }
+    }
+
+    unsafe fn _from_vec_with_nul_unchecked(v: Vec<u8>) -> Self {
         Self { inner: v.into_boxed_slice() }
     }
 
@@ -778,7 +788,7 @@ impl CString {
             Some(nul_pos) if nul_pos + 1 == v.len() => {
                 // SAFETY: We know there is only one nul byte, at the end
                 // of the vec.
-                Ok(unsafe { Self::from_vec_with_nul_unchecked(v) })
+                Ok(unsafe { Self::_from_vec_with_nul_unchecked(v) })
             }
             Some(nul_pos) => Err(FromVecWithNulError {
                 error_kind: FromBytesWithNulErrorKind::InteriorNul(nul_pos),
@@ -811,7 +821,7 @@ impl ops::Deref for CString {
 
     #[inline]
     fn deref(&self) -> &CStr {
-        unsafe { CStr::from_bytes_with_nul_unchecked(self.as_bytes_with_nul()) }
+        unsafe { CStr::_from_bytes_with_nul_unchecked(self.as_bytes_with_nul()) }
     }
 }
 
@@ -928,7 +938,7 @@ impl From<Vec<NonZeroU8>> for CString {
             };
             // SAFETY: `v` cannot contain null bytes, given the type-level
             // invariant of `NonZeroU8`.
-            CString::from_vec_unchecked(v)
+            Self::_from_vec_unchecked(v)
         }
     }
 }
@@ -1225,7 +1235,7 @@ impl CStr {
         unsafe {
             let len = sys::strlen(ptr);
             let ptr = ptr as *const u8;
-            CStr::from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr, len as usize + 1))
+            Self::_from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr, len as usize + 1))
         }
     }
 
@@ -1268,7 +1278,7 @@ impl CStr {
             Some(nul_pos) if nul_pos + 1 == bytes.len() => {
                 // SAFETY: We know there is only one nul byte, at the end
                 // of the byte slice.
-                Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
+                Ok(unsafe { Self::_from_bytes_with_nul_unchecked(bytes) })
             }
             Some(nul_pos) => Err(FromBytesWithNulError::interior_nul(nul_pos)),
             None => Err(FromBytesWithNulError::not_nul_terminated()),
@@ -1297,12 +1307,19 @@ impl CStr {
     #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
     #[rustc_const_stable(feature = "const_cstr_unchecked", since = "1.59.0")]
     pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
+        // We're in a const fn, so this is the best we can do
+        debug_assert!(!bytes.is_empty() && bytes[bytes.len() - 1] == 0);
+        unsafe { Self::_from_bytes_with_nul_unchecked(bytes) }
+    }
+
+    #[inline]
+    const unsafe fn _from_bytes_with_nul_unchecked(bytes: &[u8]) -> &Self {
         // SAFETY: Casting to CStr is safe because its internal representation
         // is a [u8] too (safe only inside std).
         // Dereferencing the obtained pointer is safe because it comes from a
         // reference. Making a reference is then safe because its lifetime
         // is bound by the lifetime of the given `bytes`.
-        unsafe { &*(bytes as *const [u8] as *const CStr) }
+        unsafe { &*(bytes as *const [u8] as *const Self) }
     }
 
     /// Returns the inner pointer to this C string.
@@ -1566,7 +1583,7 @@ impl ops::Index<ops::RangeFrom<usize>> for CStr {
         // byte, since otherwise we could get an empty string that doesn't end
         // in a null.
         if index.start < bytes.len() {
-            unsafe { CStr::from_bytes_with_nul_unchecked(&bytes[index.start..]) }
+            unsafe { CStr::_from_bytes_with_nul_unchecked(&bytes[index.start..]) }
         } else {
             panic!(
                 "index out of bounds: the len is {} but the index is {}",
diff --git a/library/std/src/ffi/c_str/tests.rs b/library/std/src/ffi/c_str/tests.rs
index 4f7ba9ad43756..00ba5460821ff 100644
--- a/library/std/src/ffi/c_str/tests.rs
+++ b/library/std/src/ffi/c_str/tests.rs
@@ -32,14 +32,6 @@ fn build_with_zero2() {
     assert!(CString::new(vec![0]).is_err());
 }
 
-#[test]
-fn build_with_zero3() {
-    unsafe {
-        let s = CString::from_vec_unchecked(vec![0]);
-        assert_eq!(s.as_bytes(), b"\0");
-    }
-}
-
 #[test]
 fn formatted() {
     let s = CString::new(&b"abc\x01\x02\n\xE2\x80\xA6\xFF"[..]).unwrap();